Forgot to copy the list. Sorry for the duplicates :( On Jul 24, 2012 1:35 PM, "Tristan Seligmann" wrote:

On Jul 24, 2012 12:32 PM, "Twan van Laarhoven" wrote:

...

On 2012-07-24 10:10, Christian Sternagel wrote:

...

Dear all,

with respect to formal verification of Haskell code I was wondering

whether (==)

...

...

of the Eq class is intended to be commutative (for many classes such requirements are informally stated in their description, since Eq does not have such a statement, I'm asking here). Or are there any known cases where commutativity of (==) is violated (due to strictness issues)?

Strictness plays no role for Eq, since to test for equality both sides will have to be fully evaluated.

I don't think this is necessarily true. For example:

(==) :: (Eq a) => Maybe a -> Maybe a -> Bool Nothing == Nothing = True Nothing == Just _ = False Just _ == Nothing = False Just x == Just y = x == y

This particular example is still commutative, however (at least I think it is).